This invention relates to sound reproduction systems involving the decoding of a stereophonic pair of input audio signals into a multiplicity of output signals for reproduction after suitable amplification through a like plurality of loudspeakers arranged to surround a listener, as well as the encoding of multichannel material into two channels.
The present invention concerns an improved set of design criteria and their solution to create a decoding matrix having optimum psychoacoustic performance in reproducing encoded multichannel material as well as standard two channel material. This decoding matrix maintains high separation between the left and right components of stereo signals under all conditions, even when there is a net forward or rearward bias to the input signals, or when there is a strong sound component in a particular direction, while maintaining high separation between the various outputs for signals with a defined direction, and non-directionally encoded components at a constant acoustic level regardless of the direction of the directionally encoded components of the input audio signals. The decoding matrix includes frequency dependent circuitry that improves the balance between front and rear signals, provides smooth sound motion around a seven channel version of the system, and makes the sound of a five channel version closer to that of a seven channel version.
Additionally, this invention concerns an improved set of design criteria and their solution to create an encoding circuit for the encoding of multi-channel sound into two channels for reproduction in standard two channel receivers and by matrix decoders.
The present invention is part of a continuing effort to refine the encoding of multichannel audio signals into two separate channels, and the separation of the resulting two channels back into the multichannel signals from which they w ere derived. One of the goals of this encode/decode process is to recreate the original signals as perceptually identical to the originals as possible. Another important goal of the decoder is to extract five or more separate channels from a two channel source that was not encoded from a five channel original. The resulting five channel presentation must be at least as musically tasteful and enjoyable as the original two channel presentation.
The derivation of suitable variable matrix coefficients and the variable matrix coefficients themselves have been improved. To assist the understanding of these improvements, this document makes reference to U.S.
U.S. Pat. No. 4,862,502 (1989) (referred to in this document as the xe2x80x9c""89 patentxe2x80x9d);
U.S. Pat. No. 5,136,650 (1992) (referred to in this document as the xe2x80x9c""92 patentxe2x80x9d); U.S. patent application Ser. No. 08/684,948, filed in July 1996 (now issued U.S. Pat. No. 5,796,844 (1998)) (referred to in this document as the xe2x80x9cJuly ""96 applicationxe2x80x9d); and U.S. patent application Ser. No. 08/742,460 (now issued U.S. Pat. No. 5,870,480 (1999)) (referred to in this document as the xe2x80x9cNovember ""96 applicationxe2x80x9d). Commercial versions of the decoder based upon the November ""96 application will be referred to in this document as xe2x80x9cVersion 1.11xe2x80x9d or xe2x80x9cV1.11xe2x80x9d. Some further improvements were disclosed in Provisional Patent Application No. 60/058,169, filed September 1997 (referred to in this document as xe2x80x9cVersion 2.01xe2x80x9d xe2x80x9cor V2.01.xe2x80x9d Further, Versions V1.11 and V2.01, and the decoders presented in this application will be referred to in this document collectively as the xe2x80x9cLogic 7(copyright) decoders.xe2x80x9d Additionally, the following are referenced in this application: [1] xe2x80x9cMultichannel Matrix Surround Decoders for Two-Eared Listeners,xe2x80x9d David Griesinger, AES preprint #4402, October, 1996, and [2] xe2x80x9cProgress in 5-2-5 Matrix Systems,xe2x80x9d David Griesinger, AES preprint #4625, September, 1997.
An active matrix having certain properties that maximize its psychoacoustic performance has been realized. Additionally, frequency dependent modifications of certain outputs of the active matrix have also been realixed. Further, active circuitry that encodes five input channels into two output channels is provided that will perform optimally with the decoders presented in this application, standard two channel equipment, and industry standard Dolby(copyright) Pro-Logic(copyright) decoders.
The active matrix decoder has matrix elements that vary depending on the directional component of the incoming signals. The matrix elements vary to reduce the loudness of directionally encoded signals in outputs that are not involved in producing the intended direction, while enhancing the loudness of these signals in outputs that are involved in reproducing the intended direction, while at all times preserving the left/right separation of any simultaneously occuring input signals. Moreover, these matrix elements restore the left/right separation of decorrelated two channel material, which has been directionally encoded, by increasing or decreasing the blend between the two inputs. For example, restoration is achieved using stereo width control. In addition, these matrix elements may be designed to preserve the energy balance between the various components of the input signal, as much as possible, so that the balance between vocals and accompaniment is preserved in the decoder outputs. As a consequence, these matrix elements preserve both the loudness and the left/right separation of the non-directionally encoded elements of the input sound.
Additionally, the decoders may include frequency dependent circuits that improve the compatibility of the decoder outputs when standard two channel material is played, that convert the inputs into two surround outputs (a five channel decoder) or four surround outputs (a seven channel decoder), and that modify the spectrum of the rear channels in a five channel decoder so that the sound direction is perceived to be more like the sound direction produced by a seven channel decoder.
The encoders mix five (or five full-range plus one low frequency) input channels into two output channels so that the energy of that input is preserved in the output when the input level of a particular input is strong; the direction of a strong input is encoded in the phase/amplitude ratio of the output signals; the strong signals can be panned between any two inputs of the encoder, and the output will be correctly directionally encoded. In addition, decorrelated material applied to the two rear inputs of the encoder will be encoded into two output channels so that the left/right separation of the inputs will be preserved when the encoder output is decoded by the decoders presented in this document; in-phase inputs will produce a two channel output that will be decoded to the rear channels of the decoders presented in this document and decoders using the Dolby(copyright) standard; anti-phase inputs will produce outputs that will be decoded as a non-directional signal when decoded by the decoders presented in this document or by decoders using the Dolby(copyright) standard; and low level reverberant signals applied to the two rear inputs of the encoder will be encoded with a 3 dB level reduction.